This invention relates to transcutaneous oxygen probes used to sense and measure the amount of oxygen emitted at the skin surface of a living body. More specifically, the invention relates to such probes which have a surface permeable to oxygen which is adapted to engage the skin of the body and which have a heat conducting member and a means for heating the member in order to warm the skin for enhancing vasodilation of the blood vessels beneath the skin and thereby increase the degree of local blood circulation and local oxygen emission.
It is known in the medical art of non-invasive blood oxygen content monitoring and measurement to apply to the surface of the skin of the person whose blood oxygen content is to be monitored and measured, a probe having a barrier permeable to oxygen and impermeable to other gases soluble in an electrolyte solution stored above the membrane. In such a device, often referred to as a Clark electrode, a small voltage is applied between two electrodes having a gap which is bridged by the electrolyte solution and the current flow between the electrodes resulting from the ionization of the solution by the dissolved oxygen is measured. The magnitude of the current is directly proportional to the amount of oxygen escaping from the blood and through the skin at the region where the probe is applied.
It is also known in the prior art to enhance the sensitivity and accuracy of the measurement of oxygen contained in the blood by using a heating device to warm the skin in the region of application of the probe to promote vasodilation of the local blood vessels thereby increasing blood flow to the region of application and also increasing the percentage of blood oxygen emitted for sensing by the probe. U.S. Pat. No. 3,628,525 to Polanyi discloses a blood oxygenation and pulse monitoring apparatus which employs optical means to measure blood oxygen content and which includes a heating coil which heats a platen in engagement with the skin of the body to enhance vasodilation of the blood vessels to enhance the oxygen measurement.
Resistive heating devices such as heating coils have also been applied to transcutaneous oxygen probes of the Clark type to heat the skin and thereby increase the degree of transcutaneous oxygen emission. Examples of Clark type electrodes employing resistive heating means are found in U.S. Pat. No. 3,795,239 to Eberhard et al., for an electrochemical electrode with heating means, U.S. Pat. No. 3,998,212 to Reichenberger for an electrode for percutaneous polarographic measurements and U.S. Pat. No. 4,005,700 to Parker for a device for measuring blood gases. French Pat. No. 2,346,716 also discloses a Clark type electrode having a heating coil or sleeve to heat the electrode for maximizing the permeability of the skin to oxygen.
It is further known in the art that the degree of heat which must be applied to the skin in order to maintain the skin at a constant temperature, i.e. the temperature at which oxygen emission is optimal, can provide valuable information relative to blood circulation and blood pressure. It is therefore important to be able to accurately monitor and measure the amount of power which must be applied to heat the oxygen probe to maintain a constant skin temperature. The quantity of power needed to maintain constant skin temperature can be displayed or used to compute a numerical indicator of the power requirement. This is referred to as the local perfusion factor.
In order to derive a measure of the power applied to prior art heated electrodes which employ resistive elements for maintaining a desired elevated skin temperature, power must be computed from the relationship P=I.sup.2 R where P is the power applied, I is the current flowing through the coil and R is the resistance of the coil or P=E.sup.2 /R where E is the voltage drop across the heating coil. In either of the above cases, it is necessary that a voltage or current quantity be squared and then multiplied by a resistance value. As a result of the squaring operation, the ratio of the terms to be multiplied (or divided) can be very large and will often exceed the maximum ratio of the multiplacand to multiplier permissible for accurate multiplication with commercially available mulplier circuits. This limitation, inherent in transcutaneous oxygen probes employing resistive heating devices, is believed responsible for the failure of such devices to take advantage of the valuable information available from the computation of local perfusion factor based on the quantity of power needed to maintain a constant probe temperature.